The present invention relates to the fabrication and processing of plastic film, and is more specifically directed to stabilizers for maintaining a tubular extrusion of blown plastic film, that is, a bubble, on a predetermined path during the extrusion process. The invention is more particularly concerned with improvements to an external guide or stabilizer arrangement that minimizes or eliminates problems caused by friction or film distortion as the extruded thermoplastic material is drawn through the guide stabilizer or sizing cage.
Polyethylene film or films of other thermoplastic materials are produced using a blown film process, in which molten thermoplastic material is fed to an annular extrusion die and the latter produces a tubular extrusion, i.e. a bubble, of the material, which is drawn upwards and solidifies into film. An air jet incorporated in the die injects air into the interior of the extrusion to inflate the bubble or tube. The film in the extrusion is initially somewhat fluid, and inflates to a larger diameter, with a correspondingly thinner wall, cooling and solidifying when a predetermined thickness is reached. The process is controlled and adjusted so that the film has a uniform thickness, and this basic process is well known in the art. The tubular extrusion is drawn upwards and flattened between a pair of rollers, i.e., nip rollers, that are positioned above the die. Typically, a collapsing frame is provided to urge opposite sides of the tubular extrusion towards one another just before the tube reaches the nip rollers. The collapsing frame may consist of a pair of opposed arrays of horizontal collapsing boards of any of a number of well-known configurations, and one arrangement is discussed in U.S. Pat. No. 4,943,226, granted Jul. 24, 1990. The film exits the nip rollers as multiple-thicknesses of film, and the film proceeds from there to various cutting, printing, rolling, or other equipment.
As mentioned above, it is important that the thickness, as well as strength and other properties, of the produced film be as uniform as possible. It is also necessary to the quality of the film that it be relatively free of weak regions and tears. For those reasons, stabilizer arrangements are used to keep the tubular extrusion confined to a well-defined travel path as it proceeds from the die to the nip rollers. Guide cages or other external stabilizers can be positioned outside the bubble for this purpose. In some cases internal guide members can disposed atop the extrusion die and within the bubble, both to prevent the tubular extrusion from drifting off the axis of travel, and also to assist in the direction of air flow within the bubble from the die.
At the current time, an external stabilizer takes the form of a series of banks of bowed arms positioned at intervals around the tubular extrusion. In one arrangement that is often used, each arm carries a row of rollers made of PTFE (Teflon). The arms are positioned just out of contact with the bubble, or else in light contact. When the bubble or extrusion drifts laterally, i.e., away from its vertical axis, the film contacts the arms and rollers on that side. As the extrusion moves past the arms, the rollers turn, and also push the extrusion back towards the vertical axis. Unfortunately, the extruded plastic is highly abrasive, so that when the bubble moves vertically past the rollers at high speed, it tends to wear the rollers flat on one side. Also, because the rollers are positioned along a curve, the adjacent rollers touch on the bubble side, but are spaced from one another on the outer side of the arms. Therefore, there is a tendency for the rollers to pinch the film as it moves past, creating snags and tears. This is especially the case where the rollers exhibit wear and so that material has eroded away. Furthermore, it is difficult and expensive to replace worn PTFE rollers, and replacement typically involves replacing all the rollers along the entire bowed arm. In addition, the materials and construction involved make the external stabilizer of this design rather expensive.
A recognized problem in the blown film art is that blown film has different stretch and strength characteristics in the axial or machine direction from what is achieved in the transverse direction, i.e., circumferentially around the extrusion. This occurs because is has been difficult to control the expansion of the bubble in both directions at the same time. Ideally, the strength in the machine direction, or MD, should be the same as the strength in the transverse direction, or TD. Stabilizing the position of the extrusion or bubble would help in this regard. Also, controlling rotation of the bubble as it rises will affect these properties.